Electrolytic fuel cells commonly operate using a phosphoric acid electrolyte under operating conditions of close to 200.degree. C. Phosphoric acid appears to be an ideal electrolyte since it is characterized by a low IR drop. While other acids may be stable at 200.degree. C., such acids are not ideal. For example, the fluorinated sulfonic (or phosphonic) acids have one advantage over phosphoric acid in that the former acids are less strongly adsorbed on the platinum-metal or alloyed catalyst used for both oxygen reduction and hydrogen oxidation within the fuel cell. Furthermore, the fluorinated sulfonic (or phosphonic) acids appear to have a higher oxygen and hydrogen solubility, a consequence of which is that oxygen is reduced more efficiently and the hydrogen electrode is more tolerant to carbon monoxide and other impurities in the fuel gas stream within the cell. These characteristics of the fluorinated sulfonic (or phosphonic) acids would suggest that these acids as electrolytes would allow cell operation at higher potentials, at the same current density, compared to phosphoric acid, thus apparently providing the higher system efficiency, or alternatively higher current densities at the same potential, yielding a lower system capital cost.
However, the fluorinated sulfonic (or phosphonic) acids exist as hydrates under the conditions of a fuel cell operating at utility conditions, in which the hydronium ions are relatively immobile. Thus, this results in low conductivity, leading to high internal cell IR drop. This disadvantage virtually annuls the improved electrode performance due to the higher solubility of the gases in these acids.
Attempts to alleviate this problem with the sulfonic or phosphonic acids as electrolytes have involved combining the fluorinated sulfonic acids either in a low molecular weight form, or in the form of polymers (such as Nafion.RTM.) with phosphoric acid. This leads to compromised properties between the two materials (the sulfonic acids and the phosphoric acid) for ionic conductivity, while approximately retaining the higher activity of the sulfonic acid as an oxygen reduction catalyst, at least for mole percentages up to about 50% phosphoric acid. However, a lowered conductivity of the mixture still does not apparently yield a major improvement in practical cell performance.
It is therefore an object of the present invention to provide an electrochemical fuel cell which, operating with phosphoric acid as the electrolyte, comprises electrodes which separate the functions of the electrolyte for gas reaction, and for ion transport, by the use of polymer perfluoro-acids.